1. Field of the Invention
This invention relates to an article, e.g. a transparency having a configured heatable member, e.g. an electrically conductive member applied to a surface of the article, and more particularly, to a heatable automotive transparency, e.g. a windshield having electrically conductive coating segments. The configured conductive member uniformly heats the surface of the article when the conductive member is energized.
2. Discussion of the Technology
Automotive heatable windshields, e.g. of the type disclosed in U.S. Pat. No. 4,820,902, include two glass sheets laminated together by a plastic interlayer, usually a sheet of polyvinyl butyral (“PVB”). A pair of spaced bus bars between the glass sheets are in electrical contact with an electrically conductive member. One type of conductive member is a sputtered electrically conductive coating of the type disclosed in European Patent Application No. 00939609.4, applied to a major surface of one of the glass sheets, and another type of conductive member includes a plurality of electrically conductive filaments of the type disclosed in U.S. Pat. No. 5,182,431. Each of the bus bars is electrically accessible by an external lead to pass current from a power source through the bus bars and the conductive member to electrically heat the conductive member to conductively heat the inner and outer surfaces of the windshield. The heated windshield surfaces attain a temperature sufficient to remove fog, and melt snow and ice. As can be appreciated, heatable windshields are practical, and in some geographical areas, heatable windshields are a requirement especially during the winter season.
Usually, the automotive windshield has a generally trapezoidal peripheral shape, and the outer major surface of the windshield as mounted in the automobile is convex with the upper portion of the windshield having the shorter length. The conductive member, usually a conductive sputtered coating between the sheets of the windshield, follows the peripheral outline of the windshield and is spaced from the peripheral edges of the sheet on which it is applied. Because of the mounted position of the trapezoidal shaped windshield, the coating is between a pair of spaced bus bars of different lengths. More particularly, the top bus bar has a shorter length than the bottom bus bar to follow the configuration of the windshield and the surface area of the windshield to be heated.
A limitation of the presently available windshields is the difference in the watt density between the coating at the shorter top bus bar and the coating at the longer bottom bus which results in non-uniform heating of the windshield surfaces and reduced efficiency in the removal of fog, ice and/or snow at the bottom portion of the windshield.
U.S. Pat. Nos. 3,789,191; 3,789,192; 3,790,752; 3,794,809; 4,543,466, and 5,213,828 present a general discussion on heatable windshields.
The non-uniform heating of the windshield surface is also present when the coating has a communication window. The communication window is provided to, among other things, pass frequencies of the electromagnetic spectrum through the conductive member. The frequencies can be in the radio frequency (RF) range to collect information from the interior of the vehicle, e.g. transit pass number at tollbooths and/or can be in the infrared range or visible range to activate a device, e.g. a rain sensor and/or an optical device. The area within the communication window has a higher resistance than the coated area surrounding the communication window because the area within the communication window has no coating or the coating is partially removed to pass frequencies of a selected wavelength range. The non-uniform heating around the communication window is observed when current moves through the coating to heat the windshield surfaces. The periphery of the communication window generates hot spots as a result of the higher watt density at the periphery of the communication window.
Communication windows are discussed in the ERTICO Committee report titled “Ensuring the Correct Functioning of ETC Equipment Installed Behind Metallized Windscreens: Proposed Short-term Solution” Version 2.0, October 1998, and The Engineering Society For Advanced Mobility Land Sea Air and Space report SAE J2557 titled “Surface Vehicle Recommended Practice”, Preliminary Draft, January 2000.
As can be appreciated, it would be advantageous to provide a heatable article, e.g. a heatable windshield that does not have the limitations of the presently available heatable windshields; more particularly, to provide an electrically conductive member that when energized uniformly heats the surfaces of a windshield, with or without an area to pass frequencies of the electromagnetic spectrum.